Download ES3210 - Gold Deposits

ES3210 - Gold Deposits
Stephen J. Piercey
ES 3210 ECONOMIC MINERAL
DEPOSITS
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Relevant Chapters in Mineral Deposits of Canada:
⇨Dube & Gosselin, Greenstone-Hosted Quartz-Carbonate Vein
Deposits ☑
⇨Taylor - Epithermal Gold Deposits (out of date, but…)
•
Optional
⇨Hart, Reduced Intrusion-Related Au Systems
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Optional
DEFINITIONS
Original terminology (archaic) by Lindgren (1933):
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EPITHERMAL
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MESOTHERMAL
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Shallow depth, 50°-200°C, moderate P
Intermediate depth, 200°-300°C, high P
HYPOTHERMAL
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Great depth, 300°-500°C, very high P
DEFINITIONS
More current usage (e.g., Robb, 2005)
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EPITHERMAL
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Lower T (~50°- ~200°C)
MESOTHERMAL
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Shallow depth (<1500 m)
Intermediate depth (1500-4500 m)
T ~200°- ~400°C
[HYPOTHERMAL]
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Great depth (>4500 m )
T ~400°-~600°C
COMMON Au-BYPRODUCT
DEPOSITS
Au is often produced as a byproduct from base metal deposits,
especially:
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VMS
Porphyry Cu-(Au)
Ni-Cu Magmatic Sulphide (e.g. Sudbury)
⇨Byproduct Au accounts for ~5-10% of current world
production (Misra, 2002)
PRIMARY AU DEPOSITS - CLASSES
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(Shallow) Intrusion-Related (incl. “RIRG”)
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CLOSELY RELATED TO THE PORPHYRY-Cu, SKARN, MANTO,
BRECCIA CONTINUUM
Quartz-Pebble Conglomerate-Hosted Paleo-Placers
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ASSOCIATED WITH URANIUM (WITWATERSRAND AND ELIOT
LAKE)
Banded Iron Formation (BIF) –Hosted
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MANY ARE A SUB-TYPE of orogenic Au
Young Placer Deposits
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FLUVIAL RECONCENTRATION OF AU FROM A WIDE VARIETY
OF CRUSTAL SOURCES
WITSWATERSRAND/
ELIOT LAKE
VMS
EPITHERMAL Au
OROGENIC
PORPHYRY
after Dubé & Gosselin, 2007
PRIMARY AU DEPOSITS - CLASSES
Next few lectures will emphasize these three most important
types:
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“Orogenic” = GQC = [“Mesothermal Lode” ]
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GREENSTONE-HOSTED QUARTZ-CARBONATE VEIN
(GQC) DEPOSITS (Piercey says “I hate this term.”)
Volcanic-Associated Epithermal
Carlin-type Sediment-Hosted Epithermal
Orogenic Gold Deposits
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Also known as (aka):
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GREENSTONE-HOSTED QUARTZ CARBONATE (GQC)
OROGENIC
MESOTHERMAL
LODE GOLD
SHEAR ZONE-RELATED QUARTZ- CARBONATE
“GOLD-ONLY” DEPOSITS
DEFINITIONS
Metamorphic water (metamorphic fluids):
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Produced by metamorphic dehydration (and
decarbonation) reactions
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Example: Chlorite (12% H2O)
Amphibole (2% H2O)
Alternatively, any fluid that equilibrated with metamorphic
rocks at T > 300 ºC
White, 1974; Skinner, 1979
Orogenic Gold
Orogenic gold are characterized by:
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Quartz-carbonate veins with valuable amounts of Au & Ag
Localized within faults & shear zones
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Generally splays (i.e., secondary or tertiary) structures
of major regional structures
DEFINITIVE CHARACTERISTICS
(LOCATION)
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Hosted by greenschist- to amphibolite-facies rocks (formed
at implied 5-10 km depth)
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Dominantly mafic composition. However, more and
more deposits are being found in other types of rocks
(e.g. Metasedimentary)
Generally found in:
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Greenstone belts (predominantly Archean)
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Ultramafic belt (e.g., Kerr-Addison; Superior Province) in
listwanite (carbonated ultramafic rock)
Slate belts (e.g., Meguma, Muruntau, Otago) (postArchean)
after Dubé & Gosselin, 2007
L. Montessi 2005
Timmins, Ontario – host to many famous orogenic Au deposits
Dome Mine, Timmins
St. Ives Gold Camp, Yilgarn Craton
Victory Mine et al. (Australia)
http://www.hughbrown.com
Can also be found as cluster of deposits
Secular Distribution: Orogenic Gold
Groves et al. (2005)
DEFINITIVE CHARACTERISTICS
(STRUCTURE)
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Structurally controlled epigenetic deposits hosted in
deformed metamorphosed terranes
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Mineralization is syn- to late-deformation. Typically postpeak greenschist facies or syn-peak amphibolite facies
metamorphism.
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Simple to complex networks of Au-bearing, laminated
quartz-carbonate fault-fill veins
after Dubé & Gosselin, 2007
DEFINITIVE CHARACTERISTICS
(STRUCTURE)
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Localized in moderately/steeply dipping, compressional
brittle-ductile shear zones & faults
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Associated shallow-dipping extensional veins &
hydrothermal breccias
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Deposits distributed along major compressional to transtensional crustal-scale fault zones in deformed greenstone
terranes
Structurally complex!!
after Dubé & Gosselin, 2007
Athena Deposit, Yilgarn, Australia
Source: https://www.goldfields.co.za
DEFINITIVE CHARACTERISTICS
(FLUID SOURCE)
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Typically associated with Fe-carbonate alteration (calcite,
dolomite, siderite, ankerite)
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Genetically associated with low salinity (typically < 3 wt%
NaCl equiv.), CO2-H2O-rich hydrothermal fluids (+ CH4, N2,
K, S, Na)
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“Chameleon” deposit -> alteration minerals tend to inherit
their chemistry from surrounding host rocks
Mainly metamorphic fluids
after Dubé & Gosselin, 2007
Mineral Assemblage
Quartz-­‐Carbonate
± Pyrite-­‐Pyrrho*te-­‐Albite in “reduced” systems ± Magne*te-­‐Hema*te-­‐Pyrite-­‐Kspar in “oxidised” systems “Oxidised”)
“Reduced”(
Bt(Amp$
Qtz(Ab(Carb$
Po$
5cm$
1cm$
Qtz$
Hem$
1cm$
St. Ives Gold Camp, Australia
HOST ROCKS
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May be hosted by all lithologies present in the local
environment, especially
⇨ mafic & ultramafic volcanic rocks
⇨ Fe-rich tholeiitic gabbroic sills
⇨ granitoid intrusions (Archean) - “porphyries”
⇨ felsic volcaniclastic and sedimentary rock of “successor
basins”
Dubé & Gosselin, 2007
HOST ROCKS
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Occasionally deposits are hosted by and/or centred within
(or next to) felsic intrusive complexes
⇨syenite porphyry complex (Kirkland Lake)
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District-specific lithological associations, implying
chemical and/or structural traps for the fluid , e.g.,
⇨ Golden Mile Dolerite Sill (Kalgoorlie)
⇨ Balmer Basalt (Red Lake)
Dubé & Gosselin, 2007
HOST ROCKS – BIF
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Many deposits referred to as “Banded Iron Formation (BIF)–
hosted” type are likely GQC deposits localized in a BIF host
lithology
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For example, this is probably the origin of the famous
Homestake Mine deposit in Black Hills, SD
Homestake Mining Company
From 1876-2002 Homestake Gold Mine
produced 40 M troy ounces of Au
1244 T of Au
A wide variety of host rocks are observed for orogenic deposits. On a
global basis, the common theme is the structural control rather than a
lithological one.
Dubé & Gosselin, 2007
GOLD HOST
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Au predominantly confined to quartz-carbonate vein
networks (mainly contained in silicates and sulphides)
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Significant Au often present within Fe-rich sulfidized
wallrock selvages, or silicified and arsenopyrite-rich
replacement zones
after Dubé & Gosselin, 2007
VEIN TEXTURES
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Moderately to steeply dipping, shear zone-hosted,
laminated fault-fill, quartz-carbonate veins in brittle-ductile
shear zones
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With, or without, fringing shallow-dipping extensional
veins & breccias
Dubé & Gosselin, 2007
Arrays of extensional quartz veins,
Pamour Mine, Timmins
Laminated fault-fill veins,
Pamour Mine, Timmins
Extensional quartz-tourmaline “flat-vein"
showing multiple stages of mineral growth
perpendicular to vein walls, Sigma Mine
(from Poulsen et al, 2000)
Dubé & Gosselin, 2007
VEIN MORPHOLOGY
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Au-bearing shear zones and faults commonly display
complex geometries
⇨ Anastomosing and/or conjugate array structures
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Individual fault-fill veins extend 10 m -100s of m
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Recognizing and delineating arrays of auriferous veins
(“ore shoots” ) represents a critical element in defining &
exploiting richest part of a orogenic Au orebody
An entire vein array can extend 1-2 km in its longest
(vertical) dimension
Dubé & Gosselin, 2007
VEIN MORPHOLOGY
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Stockworks and hydrothermal breccias may also represent
a main host to the mineralization
➡ especially when developed in competent units such as
granophyric facies of gabbroic sills (e.g. San Antonio
Mine, MB; Argo Mine, Australia)
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Ore shoots/ore zones are commonly controlled by
intersection between:
1. different vein sets or host structures, or
2. auriferous structures and especially reactive and/or
competent rock types such as Fe-rich gabbro
Dubé & Gosselin, 2007
Athena Deposit, Yilgarn, Australia
Source: https://www.goldfields.co.za
MINERALOGY
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Main gangue minerals
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quartz + carbonate (calcite, dolomite, ankerite, siderite)
Ankerite: Ca(Fe2+,Mg)(CO3)2
Variable amounts of:
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sericite + chlorite + amphibole + biotite ± scheelite (W) ±
tourmaline (B )
Main ore minerals:
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native Au
Py ± Po ± Cpy ± Arspy ± telluride
sometime hosted in silicate (biotite, amphibole)
MINERALOGY (Continued)
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Sulphides typically constitute <10% of ore
No significant vertical mineral zoning
Arsenopyrite often is the main sulphide in terranes of
amphibolite facies
(e.g., Con, Giant and Campbell-Red Lake deposits)
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Trace molybdenite or sellurides appear in some deposits
(e.g., syenite-hosted Kirkland Lake deposits)
VG
VG
Syenite-hosted high-grade quartz veins. These contain visible gold (VG),
disseminated pyrite and trace tellurides (Main Break, Kirkland Lake, ON).
Dubé & Gosselin, 2007
High-grade zone.
quartz carbonate vein. Visible
Au & Asp-rich replacement of
host basalt.
Red Lake Mine, ON
Arsenopyrite-rich auriferous silicification overprinting low grade to barren
carbonate±quartz veins is the main host association of Campbell-Red Lake
(Goldcorp) deposit.
(This deposit is in amphibolite facies terrane).
Dubé & Gosselin, 2007
Dubé & Gosselin, 2007
C. “Green-carbonate rock” showing fuchsite-rich replacement with Fecarbonate veining in highly deformed ultramafic rocks, Larder Lake.
D. “Green-carbonate” alteration showing abundant green micas replacing
chromite-rich ultramafic rocks, Baie Verte, NL.
Chromium-rich micas (esp. fuchsite) are common indicators of potential
orogenic Au mineralization in ultramafic terranes.
Fuchsite K(Al,Cr)3Si3O10(OH)2 [Cr-Muscovite]
ALTERATION - GREENSCHIST
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Altered host rocks proximal to veins show
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Enrichment in CO2, S and K2O
Depletion of Na2O
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Further from veins, alteration characterized by chlorite +
calcite (± magnetite)
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Dimensions of alteration haloes vary widely with
composition of the host rocks (m - km)
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Haloes may fully envelope entire deposits where hosted by
mafic and ultramafic rocks
Dubé & Gosselin, 2007
ALTERATION - AMPHIBOLITE
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Alteration may be more complex & varied in amphibolite
facies terranes
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Common assemblages associated with Au mineralization
include:
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biotite, amphibole, pyrite, pyrrhotite & arsenopyrite
biotite/phlogopite, diopside, garnet, pyrrhotite ±
arsenopyrite ± K-feldspar ± calcite ± clinozoisite
Dubé & Gosselin, 2007
ALTERATION - AMPHIBOLITE
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Canadian examples of amphibolite facies deposits
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Madsen-Red Lake (Dubé et al 2000, 2001b)
Eau Claire-James Bay (Cadieux, 2000)
GEOCHEMICAL SIGNATURE OF ORE
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Orogenic gold ore is enriched in:
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As, Sb, W, Mo, B and Te
Typically relatively low concentrations of:
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Au, Ag
Cu, Pb and Zn
No vertical metallic zoning in ore shoots
Au : Ag ratio > 1; Typically 5 to 10
Dubé & Gosselin, 2007
TRANSPORT OF GOLD
Reduced sulphur complex
at near neutral pH
(From Mickuki, 1998)
Another look at Au
Put these two slides in as they are
useful. Use them if you see fit.
AuCl2- associated with what types of assemblages?
Au(HS)2- associated with what types of assemblages?
From Williams-Jones et al. (2009)
Au Solubility
So, how do we get gold to precipitate?
From Williams-Jones et al. (2009)
PRECIPITATION OF GOLD
2 principal methods:
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Wallrock sulphidation (Mikucki, 1998 and Williams-Jones et
al. (2009)
FeOrock + 2H2S = FeS2 + H2O+H2
and
Au(HS)2-+H++1/2H2 = Au + 2H2S
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Earthquake
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Rapid expansion in fracture could cause the fluids to
evaporate, triggering almost instantaneous gold deposition
- seismic pumping (Sibson et al., 1987; Craw, 2013)
Genetic Model - Orogenic Au
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Ore-forming fluids are typically:
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1.5 ± 0.5 kbar
350° ± 50°C
Low-salinity
H2O-CO2 ± CH4 ± N2
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Gold predominantly transported as a reduced sulfur complex
(Mikuki, 1998; Goldfarb et al., 2001; and Groves et al., 2003).
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Current models emphasize a deep source for gold and fluids related to metamorphic devolatilization.
Dubé & Gosselin, 2007
Genetic Model
Kerrich et al. (2005)
Genetic Model
From
Groves et al. (1998) and
Goldfarb et al. (2001)
Genetic Model (Review)
Transport
Deposition
Source
Location:
splay structures off larger regional fault systems
(2nd-3rd order)
Mechanisms: - wallrock sulphidation in Fe-rich rocks
- rapid P decrease (earthquake)
Au: as reduced S complexes (mostly Au(HS)2pH: near neutral (5-7)
Fluids: metamorphic fluids => low salinity, CO2-H2O-rich
Au: metamorphic fluids
Depth: deep source (5-10km)
T (°C): moderate temperature (350° ± 50°C)
COEXISTING Au DEPOSIT TYPES
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In Archean greenstone belts, other types of Au deposits
(formed at different crustal levels) have become
juxtaposed against orogenic gold deposits as a
consequence of progressive long term tectonism and
metamorphism.
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Au-rich VMS (formed earlier) or intrusion-related Au
deposits (often formed later) now co-exist along major
regional faults.
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For example, the Bousquet-LaRonde Au-rich VMS deposits
lie along the Larder Lake-Cadillac Fault, near several
orogenic gold deposits east of Noranda in the Abitibi
greenstone belt.
Greenstone-Hosted
Orogenic (quartz-carbonate vein
(GQC) vein) Deposits
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Canadian examples:
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Sigma-Lamaque (PQ)
Dome and Kerr Addison (ON)
Giant and Con (NWT)
San Antonio (MB)
Hammerdown and Deer Cove (NL)
Bralorne-Pioneer (BC)
Distribution of Canadian orogenic Au deposits with respect to structural
province
Dubé & Gosselin, 2007
Modified from Poulsen et al (2000)
* mainly in
secondary or
tertiary
structures
Simplified geological map of the Abitibi Greenstone Belt.
Orogenic Au deposits are typically associated with large scale (crustal)
transpersonal faults.
⇨ Note also the spatial association with Au-rich VMS deposits
Dubé & Gosselin, 2007
TROY OUNCES & GRAMS
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Au commodities price is commonly quoted in $US per troy
ounce:
1 Troy Ounce = 31.1 g
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At $1230/oz, 1g of Au is worth US$40
Ore that is 1g/t of Au contains the equivalent of only 1ppm
Au
WORLD PRODUCTION & RESERVES
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Historical production + reserves for the entire orogenic
gold deposit subtype is ~16,585 T Au (Dubé and Gosselin,
2004) - equivalent to 13 % of the world total for Au
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41 “world-class” orogenic gold deposits contain >100 T of
Au - including 12 giant deposits with > 250 T
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7 of these 41 are from the Archean Superior Province - 6
from the Abitibi GB and 1 from the Uchi Sub-Province
(Campbell-Red L. Deposit)
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Superior Province is the single largest well-preserved
Archean craton in Au endowment, followed by the Yilgarn
craton of Australia
WORLD PRODUCTION & RESERVES
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Historical production + reserves for the entire orogenic Au
deposit subtype
~16,585 T Au (Dubé and Gosselin, 2004)
⇨ Equivalent to 13 % of the world’s total historical production of Au
⇨ US Government reports a holding of 8,133.5 tonnes as a bullion
reserve
(June 2013 - World Gold Council data)
Dubé & Gosselin, 2007
1 oz/T
Tonnage and grade for all global Au deposits containing >30 T Au
WORLD PRODUCTION & RESERVES
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At 13% of historical production, orogenic gold are second
only to the Witwatersrand “paleoplacers” of South Africa
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Orogenic gold deposits typically 5 - 15 g/t Au
Tonnage highly variable (104 - >107 T ) ⇨ typically a few
million T of ore
WORLD PRODUCTION & RESERVES
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Largest greenstone orogenic gold deposit (total Au
content) is the Golden Mile complex, Kalgoorlie, W.
Australia (Norseman-Wiluna GB, Yilgarn Block)
⇨ 1821 T Au
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The Hollinger-McIntyre deposit in Timmins, ON (Abitibi GB)
is second largest orogenic gold known
⇨ 987 T Au
Dubé & Gosselin, 2007
Tonnage vs grade of Canadian and all world-class size (≥100 T Au)
orogenic deposits.
GRADE AND TONNAGE
CHARACTERISTICS
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In Canada, the discovery (Slave, Yellowknife GB) and
Campbell-Red Lake (Superior, Uchi –Red Lake GB)
deposits have had the highest average grades ⇨ 34 g/T &
23 g/T Au, respectively
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The Goldcorp high-grade zone of the Campbell-Red Lake
deposit has an average production grade of 88 g/T Au
since mining began (Dubé et al, 2002)
GSC, 2007
DEFINITIONS
In reality, Fluid Temperatures for Epithermal Au & Mesothermal Au
environments largely overlap (100 - 400ºC)
⇨ Depth (P) Regimes are a more fundamental difference between GQC
& other Au deposit types – as is fluid chemistry
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EPITHERMAL ⇦ Epithermal Au
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Shallow depth (<1500m)!!
Lower T (~50°- ~200°C)
MESOTHERMAL ⇦ Orogenic (GQC) Au
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Intermediate depth (>1500m)
T ~200°- ~400°C
Distribution of Canadian Orogenic Au deposits
⇨ by structural province
Dubé & Gosselin, 2007
GSC Open
File 4668
All Au Producing Districts of Canada.
“Lode Gold” in this GSC report refers to all hydrothermal deposits whose
principal commodity is gold ⇨ i.e., orogenic & epithermal types
EPITHERMAL Au DEPOSITS
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“Epithermal” Au deposits account for < 5% of Canadian
production, but a substantially higher percentage of total
global production.
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In Canada, they occur predominantly in extensional
terranes in Mesozoic & Tertiary rocks of the Cordillera
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Important Canadian deposit examples:
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Mt. Skukum, YT
Blackdome, Cinola, Toodoggone, BC
Hope Brook, NL [Neoproterozoic - Avalon Zone]
EPITHERMAL Au DEPOSITS
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Often amenable to open-pit & to heap-leach extraction of
Au
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Almost all well-studied examples of epithermal Au
deposits are located in circum-Pacific volcanic belt
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In particular:
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Basin & Range Province, western USA
Western Pacific island arcs
Andean continental arcs
WITSWATERSRAND/
ELIOT LAKE
VMS
EPITHERMAL Au
OROGENIC
PORPHYRY
after Dubé & Gosselin, 2007
Distribution of epithermal Au deposits in the Circum-Pacific region
Hedenquist et al, 2000
EPITHERMAL Au DEPOSITS
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Individual deposits characterized by marked variations in
temperature & pressure across ore forming regime
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Ore deposition provoked by marked changes in
physicochemical conditions of ore-forming fluids over
short distances (meter scale)
⇨ Often vertically zoned
DEFINITIVE CHARACTERSITICS
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Typically hosted in calc-alkaline volcanic pile, most
commonly andesitic volcanics & pyroclastics (sub-caldera
environment)
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Often localized within fractures associated with caldera
structures
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Epigenetic mineralization in quartz veins
Vein systems flare upwards into wedge- or cone-like
features
DEFINITIVE CHARACTERSITICS
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Principal economic metals: Au & Ag
Characteristic ore-associated wallrock alteration types:
⇨ Adularia-sericite (ADS)
⇨ Quartz-kaolinite-alunite (QAL)
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Other classification based on sulfidation state (most
common):
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High sulfidation (~QAL; high sulfur)
Low sulfidation (~ADS; low sulfur)
Also have intermediate sulfidation
NB: Subsidiary (aka) terms involving “sulphur” are misleading.
Those involving “sulphidation” are widely used, and have an
explicit physicochemical meaning (i.e., sulphide assemblage).
B
Abundance of Au-Ag-base metals in major ore deposit types
Poulsen, 1996
Meaning of the term
“sulfidation”
High or low
sulfidation state
is reflected by
the stable
sulfide mineral
assemblage and
is fundamentally
controlled by T
and ƒS2
Einaudi et al, 2003
Low vs. Intermediate vs. High Sulfidation
Advanced Argillic
Sillitoe and Hedequist (2003)
A simpler table
Simmons et al. (2005)
EPITHERMAL Au - MINERALOGY
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Au typically as native Au and/or electrum (Au-Ag)
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Most have Au:Ag <1
Au sometimes in tellurides or dissolved within sulphides
(e.g., in arsenious Py)
Hot spring, Carlin-type & some others have a
characteristic Au, As, Sb, Hg, Tl association
EPITHERMAL Au DEPOSITS
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Hot spring-type
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May comprise part of high and low sulfidation types;
more common in the latter.
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Essentially any significant (subaerial) surface
expression of an epithermal system
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Near surface silicified zones, sinters or phreatic
breccias ±(Au/Ag) mineralization
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Explicitly identified by sinters (only stable in low
sulfidation systems), or other evidence of nearsurface (100ºC) boiling
MINERALOGY-Low Sulfidation
Ore minerals :
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Pyrite ±
Pyrrhotite, arsenopyrite, Fe-rich sphalerite
chalcopyrite,tetrahedrite/tennantite, Fe-poor sphalerite (galena,
acanthite (argentite))
and:
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Quartz (sugary & crystalline), chalcedony as colloform, crustiform,
cockscomb veins
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Adularia
Calcite ± rhodochrosite ± barite (often bladed in nature)
Chlorite
Fluorite
MINERALOGY- Low Sulfidation
Many low sulfidation eposits contain an ore assemblage with one or more of
the following:
Native gold
Au
Electrum
Au-Ag
Native silver
Ag
Acanthite (argentite)
Ag2S
Naumannite
Ag2Se
Aguilarite
Ag4SeS
Native silver + one or more of the Ag-Se-S minerals is the “ginguro“ (silverblack) assemblage described at Hishikari and elsewhere.
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Inner zone (silicification)
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Outer Zone (potassic-phyllic
Alteration)
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Replacement of wallrocks
by quartz/chalcedony
quartz ± Kfeldspar(adularia) ±
sericite
Distal/overprinting zone (argillic
alteration)
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Kaolinite/Smectite/
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Chlorite + carbonate ± epidote
are often present in a broad
surrounding envelope (Propylitic
Alteration)
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Sinter/steam-heated zone
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Kaolite, alunite, native S,
opaline silica.
From Hedenquist et al. (2000) and
Tosdal et al. (2009)
Crustiform Quartz-Adularia Veins
(Low Sulfidation)
M. Thirnbeck
A typical example of crustiform banded quartz-adularia. Indonesia
Etoh et al, 2002
Quartz pseudomorphs after bladed calcite, Hishikari, Japan
Bladed calcite, and quartz pseudomorphs, are commonly formed in
the boiling portions of ADS/Low-Sulfidation Au deposits
SPECIAL MINERALOGY/LITHOLOGY
Hot Spring Type Deposits
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Sinter*
* A fine-grained chemical sedimentary rock deposited by
precipitation from mineral waters, especially siliceous
sinter and calcareous sinter . May contain clays,
sulphate minerals, minor pyrite.
Silica sinter is only stable in ADS-Type fluids
Often a result of magmatic fluids/vapours interacting with
the water table.
Sinter
Crowfoot-Lewis
deposit,
Nevada
Champagne Pool, NZ
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Mud Pools, Torotoro National Park,
Bolvia
Sinters show columnar structures not typical of
other naturally occurring silicas
White et al, 1989
Native Sulfur, Alunite
Crowfoot-Lewis,
Nevada
KAl3(SO4)2(OH)6.
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Inner zone (silicification)
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Outer Zone (potassic-phyllic
Alteration)
•
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Replacement of wallrocks
by quartz/chalcedony
quartz ± Kfeldspar(adularia) ±
sericite
Distal/overprinting zone (argillic
alteration)
•
Kaolinite/Smectite/
•
Chlorite + carbonate ± epidote
are often present in a broad
surrounding envelope (Propylitic
Alteration)
•
Sinter/steam-heated zone
•
Kaolite, alunite, native S,
opaline silica.
From Hedenquist et al. (2000) and
Tosdal et al. (2009)
MINERALOGY-High Sulfidation
“High sulfidation” ore minerals:
•
•
Pyrite + enargite/luzonite + covellite ±
bornite, chalcocite, bismuthinite
and:
•
•
•
•
Quartz (“vuggy silica”)
Alunite and Al-silicate (kaolinite-dickite)
Barite ± native sulphur
Bladed calcite is NOT Characteristic
High Sulfidation deposits are
characterized by advanced argillic
alteration:
•
•
•
Quartz + kaolinite* +
alunite + dickite* + pyrite
•
•
Diaspore* may also occur
Pyrophyllite [Al2(Si4O10)
(OH)2 ], instead of kaolinite
in deeper deposits
Carbonates absent !! (very low pH)
Zones of silica replacement &
vuggy (residual) silica are common
*Kaolinite Al2Si2O5(OH)4
Triclinic
*Dickite
Monoclinic
Al2Si2O5(OH)4
*Diaspore AlO(OH)
Orthorhombic
From Hedenquist et al. (2000) and
Tosdal et al. (2009)
Henley, 1991
Ore Mineralization
classically vertically below
argillic alteration & contains
the high sulfidation ore
assemblage
Ore mineralization often
replaces pre-existing vuggy
silica
Advanced argillic alteration
forms an ore envelope
Propylitic alteration
constitutes a broad
underlying host envelope –
dominantly chlorite ±
epidote
Schematic cross-section of high sulfidation deposit. Showing
alteration, mineralogy & general location of ore zones
Vuggy Silica
Vuggy silica is a residue of
heavy leaching of the host
rocks by acid hydrothermal
fluids
Stratex – Altintepe, Turkey
geocosas.wordpress.com
Vuggy Silica, Cerro Rico, Potosi, Bolivia
SPECIAL MINERALOGY/LITHOLOGY
•
Quartz-kaolinite-alunite deposits
•
•
•
•
•
Alunite
KAl3[(OH)3|SO4]2
Natroalunite (Na,K)Al3[(OH)3|SO4]2
Jarosite
KFe3+3[(OH)3|SO4]2
Enargite
Cu3AsS4
Orthorhombic
Luzonite
Cu3AsS4
Tetragonal
Dyet.net
KAl3 [(OH)3|SO4]2
Alunite.
Marysvale, UT
mindat.org
Alunite on quartz (20mm sample)
ALUNITE
Heves Co., Hungary
Enargite and Pyrite (8mm sample).
Red Mountain Deposit, San Juan
Co., CO
Cu3AsS4
mindat.org
ENARGITE
Enargite (5mm sample).
Julcani Mine, Angaraes Pr., Peru
mindat.org
Although it is a consistent
component of many examples, not
all epithermal Au systems conform
to a simple, structurally controlled,
“stockwork feeder” geometry.
Inhomogeneities in country rock
permeability, and hydrothermally
induced porosity, can also act to
localize alteration and ore zones.
Hedenquist et al, 2000
From Hedenquist et al. (2000) and
Tosdal et al. (2009)
Epithermal Au - SIZE & GRADE
•
•
Grades typically 2.5 – 25 g/t
•
•
Au:Ag typically < 1 (unlike GQC)
Tend to be smaller than orogenic gold deposits – many
epithermal Au deposits have been exploited at < 1Mt of
ore
Some deposits contain “Bonanza” zones that have grades
well in excess of 30 g/t
Hishikari Au Deposit
Kyushu Island, Japan
5.2 Mt @ 60g/T Au
(Hedenquist et al, 2000)
Metal Mining Agency of Japan, 1990
Ultra high grade Au ore with quartz-adularia in rhyolite
breccia
Sleeper Deposit, Humboldt Cty., Nevada
Long dimension is 10.4 cm
James St. John, OSU-Newark
Reserves & production statistics-epithermal Au deposits of BC
Panteleyev, 1988
Taylor,
1996
Grade vs tonnage for Canadian epithermal Au deposits
SIZE & GRADE - High Sulfidation
•
High sulfidation deposits of magmatic-hydrothermal
origin tend to be restricted to areas proximal/above the
implied heat source
•
For example at Summitville, CO, altered rocks outcrop
over an area of 1.5 km x 1.0 km (produced 3.5 T of Au)
•
At the Al Deposit (Toodoggone) quartz-clay-alunite(+barite
+dickite) alteration is exposed over 250m x 1500m
after Taylor 1996
Schematic cross-section of the HS deposit at Summitville, CO.
Note close spatial relationship of deposit to underlying porphyry
stock, and implied position of deposit within an original resurgent
rhyolite dome within a parent caldera complex.
Hedenquist, 2008
SIZE & GRADE - Low Sulfidation
•
Low sulfidation deposits can cover very large areas, even
though the host-rock alteration is generally restricted to a
narrow envelope enclosing veins and breccias
•
Blackdome Mine (BC) - quartz veins up to 0.7m thick and
2200m long, within a 2 km x 5 km area, contain ~8.9 T Au
•
Creede, CO, - mineralized veins have been mined over
strike lengths exceeding 5 km
after Taylor 1996
Commodore Mine, Creede, CO. Circa 1900
GEOLOGICAL SETTING
•
Intraoceanic island arc volcanoes
⇨ e.g., Papua New Guinea
•
Continental arcs
⇨ e.g., Silverton & Creede calderas, CO
after Taylor 1996
STRUCTURAL CONTROLS
•
Regional strike-slip faulting
⇨ e.g., Eocene of BC
•
Features of caldera volcanism
⇨ e.g. Colorado deposits:
•
•
•
Ring fractures
Radial faults
Extensional faults from resurgent doming
after Taylor 1996
An example of
laterally extensive
veins in an low
sulfidation-type
deposit
Structural setting and vein distribution. Blackdome Mine, BC
Taylor,
1996
Thickness x grade cross-section. Cirque Vein, Mt. Skukum, YT
Taylor, 1996
Secular Distribution
Goldfarb et
al. (2010)
AGE SIGNIFICANCE?
•
Most deposits are Tertiary-age or younger
•
•
Since these are shallow phenomena, older deposits
are more likely to have been removed by erosion
However, examples of epithermal Au type mineralization
are found Late Proterozoic through Recent:
•
•
•
Toodoggone, BC – Jurassic
Queensland, AUSTRALIA – Paleozoic
Avalonian (NL) - NeoProterozoic
Genetic Models
From Hedenquist et al. (2000) and
Tosdal et al. (2009)
Epithermal Systems
From Robb (2005) after Hedenquist et al. (2000)
GENETIC MODEL-High Sulfidation
Au
•
Formed at shallow (“epithermal”) depths in the core of a
volcanic edifice
•
•
Overlie intermediate to felsic intrusions of porphyry-type
Implied dominant role of magmatic water – especially
during initial stages
GENETIC MODEL-Low Sulfidation Au
•
Formed in upper few 100 m of a large hydrothermal
system
•
System driven by volcanic heat - but dominated by
variable contributions from meteoric water (more in LS
than HS)
•
Extensive lateral fluid flow may result in ore deposition
over wider area, sometimes displaced laterally from
proposed heat source
Epithermal Systems
From Robb (2005) after Hedenquist et al. (2000)
Epithermal Systems
From Robb (2005) after Hedenquist et al. (2000)
Au Solubility
Boiling
1) Au(HS)2-+ = Au + H2S + S
2) AuCl2- +H+ = Au + 2HCl
Mixing
1) 4Au(HS)2-+ 2H2O+ 4H+ =
4Au + 8H2S + O2(g)
2) 4AuCl2- + 2H2O = 4Au +
8Cl- + 4H+ + O2(g)
From Williams-Jones et al. (2009)
Epithermal Systems
4Au(HS)2-+ 2H2O+ 4H+ = 4Au + 8H2S + O2(g)
4AuCl2- + 2H2O = 4Au + 8Cl- + 4H+ + O2(g)
From Robb (2005) after Arribas et al. (1995)
“Intermediate Sulfidation” Deposits
•
Recent coinage of intermediate sulfidation (IS) deposits
refers to a subgroup that displays many of the general
characteristics of ADS (LS) deposits.
•
IS are characterized by qz-sericite with barite,
rhodochrosite, anhydrite
•
•
•
Creede, Co is considered a typical example of IS
IS are associated with andesitic continental arc volcanism
IS are deeper (300-800m) & slightly hotter (>225°C) vs LS
(<300-400m; <225°C) (Hedenquist 2008)
Epithermal Systems
From Robb (2005) after Hedenquist et al. (2000)
Carlin-type gold deposits
READ: Robb (2005) Section 3.9.2 Carlin-type Gold Deposits, p.192-195.
Posted on web
Carlin-Type Au Deposits
•
•
Exact genesis still controversial !!
•
Significant potential for additional discoveries of this
deposit type beyond Nevada/Western USA.
Generally recognized as a separate Type of epithermal Au
deposit – but possess characteristics of both orogen and
epithermal Au deposits.
⇨ Carlin-type deposits ostensibly recognized in the 1980s in
Guizhou Province, SE China.
⇨ Belt of Au mineralization with strong Carlin-type features
currently being explored in YT.
The Carlin Deposit
•
•
•
Discovered 1961 – Newmont Mining Co
•
Numerous subsequent discoveries of this “type” in the
Carlin Trend and four related districts in Nevada
•
Other examples exist in similar terranes from N. Mexico
through Montana; also China and YT.
Production began 1965
Estimated to contain 3440 T of recoverable Au (Teal &
Jackson, 1997) @ < 5g/T
after Berger & Bagby, 1991
Carlin-type Au
deposits in Nevada
(white circles)
Hofstra et al,
2003
T. Moore, USGS(2012)
Battle Mountain Mining District, Nevada
DEFINITIVE CHARACTERISTICS
•
Most favourable host rocks:
⇨ Silty carbonaceous rocks (dirty carbonates)
•
•
Fine-grained, detritus-rich lithologies are often most susceptible to
broad scale replacement/Au-mineralization
Major host structures:
⇨ High-angle normal faults
•
•
Related to tectonic doming of autochthonous* rocks.
Important in channeling fluids/localizing mineralization
* In a tectonic context: found where they and their constituents were
formed [opposite of allochthonous = tectonically emplaced suites]
after Berger & Bagby, 1991
Schematic Cross Section: Carlin
from Ridley (2013)
Meikle Mine
from Ridley (2013)
DEFINITIVE CHARACTERISTICS
•
Granitic igneous rocks occur (or are suggested by
geophysics) in the vicinity of all the Nevada Carlin-type
deposits
•
these intrusions themselves often display intensive
hydrothermal alteration
•
overlying ore mineralization commonly occurs in
fractures parallel to the intrusions
•
to date, no direct genetic link established between
intrusions and Au deposits
•
genetic link is implied, however (e.g., Ressell et al.,
2006; Muntean et al., 2011)
after Berger & Bagby, 1991
Regional Relationships
from Ressell and Hendry (2006), Dickinson (2004) and Ridley (2013)
DEFINITIVE CHARACTERISTICS
•
Pre-Au stage jasperoid often replaces carbonate rocks
along/adjacent to fault structures
•
•
barite a common accessory of this stage
Syn-Au stage with continued silicification:
•
•
•
quartz veins and silicified breccias
additional silicification of host rocks
some clay alteration of detrital grains in hosts
after Berger & Bagby, 1991
DEFINITIVE CHARACTERISTICS
•
Post-Au calcite veins are common
•
•
±barite ±fluorite
±orpiment ±realgar ±stibnite ±cinnabar ±Tl minerals
after Berger & Bagby, 1991
DEFINITIVE CHARACTERISTICS
•
Evidence of dissolution of carbonate minerals &
precipitation of silica in host rocks
•
Au deposition associated with low-salinity, high-CO2,
high-H2S fluids (similar to orogenic fluids)
•
Available data for Au-deposition stages give 200 - 300°C
range for ore formation
•
No apparent vertical zonation in ore mineralogy (similar to
orogenic Au)
after Berger & Bagby, 1991
Schematic Cross of Carlin-type Deposit
from Hofstra and Cline (2000) and Ridley (2013)
AGE
•
Absolute age has been difficult to establish for the
Nevada deposits:
•
Likely >28Ma (Oligocene)- based on radiometric
dating of supergene alunite (Gold Quarry deposit)
•
Other deposits give radiometric ages in the
Cretaceous (<144Ma) – but these may be preserved
ages of minerals in host rocks
•
Arehart et al (2003) argue for 33-42 Ma based on
compilation of what they deem most reliable
geochronological data available
after Berger & Bagby, 1991; Arehart et al 2003
REGIONAL GEOLOGY/TECTONICS
•
Host rocks for Nevada deposits are allochthonous Paleozoic
through Early Mesozoic sedimentary rocks (+lesser volcanic
rocks)
⇨Originally deposited along complex evolving continental margin
•
Sediments have subsequently been affected by three major
orogenic events:
•
•
•
•
L. Dev.-E. Miss. Antler Orogen
(~360Ma)
L. Permian Sonoma Orogen
(~240 Ma)
L. Jurassic - E. Cretaceous Sevier Orogen (~135Ma)
Only this last (Mesozoic) orogeny was accompanied by
magmatism
after Berger & Bagby, 1991
Roberts
Mountain
Thrust
Regional Tectonic Context of the Nevada Deposits
Berger & Bagby, 1991
gray/Stipple pattern
shows extent of Paleozoic
allochthon
Roberts Mountain Rocks
MINERALOGY
•
Native Au – extremely fine-grained
⇨ often ≤1 micron (almost sub-microscopic)
•
•
±native Ag ±tellurides
Pyrite the most common sulphide
•
•
•
Py often the host for native Au
Au also commonly occurs as coatings/inclusions on/
within arsenian (As-rich) zones in Py
±marcasite ±arsenopyrite
after Berger & Bagby, 1991
Upper frame is a map of As concentration.
Bands labelled 3a-3c in Lower frame show the
deportment of Au in zones of arsenious pyrite
consequent to a specific series of
hydrothermal episodes – referred to by the
authors as the “Carlin event” for this deposit.
From Barker et al, 2009
ALTERATION
•
Highest Au grades at Carlin are associated with zones of
co-extensive silicification and K-metasomatism
characterized by:
•
quartz + dolomite + illite/sericite
•
Highest Au grades at Carlin are not closely associated
with pre-Au Jasperoid or most intensive silicification
•
Other alteration zones may include:
•
dickite, kaolinite, calcite, K-feldspar
after Berger & Bagby, 1991
Simplified regional cross-section illustrating perceived fundamental
structural and lithologic controls on the location of Carlin-type gold
deposits in Nevada
Robb 2007: after Hofstra & Cline 2000
Ck District
n Trend
??
After Heitt et al 2003
SOME PROPOSED GENETIC
MODELS
•
Distal magma-related replacement deposits (Berger &
Bagby, 1991)
•
•
•
Magmatic CO2 + H2S + H2O(+ Contact Metamorphic
CO2). Mixing with low-CO2 low-T meteoric water
causing Au ore deposition (Rose & Kuehn (1987))
Distal W (Au)-skarns (e.g. Sawkins, 1984)
SEDEX model for “stratabound” portions of Carlin
Deposits (Berger & Theodore, 2005)
after Berger & Bagby, 1991
Muntean et al.’s
Model
Muntean et al.’s
Model
A newly recognized belt of Carlin-style Au mineralization in Yukon Territory:
The Rau-Nadaleen Trend
ATAC Resources, 2013 Corporate Presentation
ATAC Resources, 2013 Corporate Presentation
OSIRIS AREA – CONRAD ZONE
ATAC Resources, 2013 Corporate Presentation
What is the single largest use of gold?
(BLING BLING)
Sector Usage of Gold – Au(T)
galmarley.com
A staggering 320 tons of gold and more than 7,500 tons of
silver are now used annually to make PCs, cell phones, tablet
computers and other new electronic and electrical products
worldwide
BullionStreet.com
Monday, July 9th, 2012
Largest Gold Consumers
galmarley.com
Largest Primary Gold Producers(2002) – Au (t)
goldsheetlinks.com
Officially Reported Gold Holdings
June 2013
•
•
•
•
•
United States
8,133.5 T
China
1,054.1 T
Russia
1,015.4 T
Japan
Canada
765.2 T
3.2 T
World Gold Council